The refraction procedures in the optometric examinations are used to identify various conditions such as refractive error of the eye. The refractive error of an eye (ametropia) is a defect in the ability of the eye to focus parallel rays of light from distant objects on the retina. A distant object is typically viewed at six meters or more from the tested eye. The object used is typically a character such as a letter, number etc. of a predefined size presented over a board or a screen. Refractive error types include myopia, hyperopia and astigmatism.
The refractive error of the eye may be evaluated using subjective or objective methods. Objective methods require a professional examiner to conduct the examination of the patient's eye, using special equipment designed to determine the patient's refractive status and prescribe the appropriate lens (glasses lens and/or contact lens) for each eye.
The subjective method requires the patient to answer specific questions regarding the clarity of a letters/targets presented on a chart at a distance of 6 meters. The examiner interposes lenses of various dioptric powers at a distance of approximately 12 mm from the patient's eye and the patient is required to discern changes in the clarity of the characters by answering forced choice questions. Typically, the patient is asked which of two lens presentations provides the better acuity.
The subjective refraction seeks to find the spherical power and the cylindrical power and axis. Most optometrists and ophthalmologists will use a cross cylinder in conjunction with a trial frame or phoropter in order to determine the cylindrical power and axis. The subjective refraction requires the examiner to interpret the patient's responses.
The objective refraction on the other hand does not require any patient responses. The patient has a passive role while his/her refractive error is evaluated by various possible objective methods and means. Auto refraction and retinoscopy are the most common objective methods. Methods such as photo refraction and aberrometry are also available and used. The less accurate objective refraction usually precedes the subjective result that is used to define the final prescription. An example of a optometric prescription in the minus cyl convention is −2.00/−1.00×90 (sphere −2.00, cyl −1.00, axis 90 deg). In other words, far cyl −2.00 at 180 deg and near cyl −3.00 at 90 deg.